Reusability determination method for gear and reusability determination system for gear

A reusability determination system for a gear during or after use, comprising:

• a measuring apparatus including a retained γ phase measurement section that measures a content of retained γ phases in the gear and a residual stress measurement section that measures residual stress applied to the gear;
• and
• a determination apparatus including a retained γ phase change rate calculation section that calculates a change rate of the content of the retained γ phases, a residual stress ratio calculation section that calculates a residual stress ratio to residual stress in an initial state of the gear, and a determination section that determines whether the gear is reusable or is not reusable, wherein
• the determination section determines, on a basis of results of the calculation in the retained γ phase change rate calculation section and the residual stress ratio calculation section, a state of the gear corresponds to which one of a first phase, a second phase, and a third phase,
• the first phase being a phase in which the change rate of the content of the retained γ phases decreases as a change rate of the residual stress increases with reference to a state of the gear at time of start of use,
• the second phase being a phase in which, after the first phase, the change rate of the residual stress increases steeply from that in the first phase as the change rate of the content of the retained γ phase decreases, and
• the third phase being a phase in which, after the second phase, the change rate of the residual stress decreases intermittently with respect to that in the second phase.